Described below are a method, system, network device and computer program product for relaying a signal via a multi-antenna relay station.
Wireless networking constitutes an important component of future information technology applications. Recently, the use of multiple antennas at wireless transmitters and receivers has been proposed as an enabling technique for high-rate multimedia transmissions over wireless channels.
For transmissions from a transmitting node (e.g., terminal device, base station, access point, etc.) to a receiving node (e.g., terminal device, base station, access point, etc.) of a wireless multi-hop network (e.g., cellular network, wireless ad-hoc network, wireless local area network (WLAN), wireless broadcast and/or broadband network, etc.) relay stations can be exploited to relay messages for end users. Motivation for using relay stations can be that, in a cellular network, direct transmissions between the base station and users close to the cell boundary can be very expensive in terms of the transmission power required to ensure reliable communications, or that existing radio frequency (RF) technologies typically can accommodate only one or two antennas at the user end, indicating that current wireless systems cannot fully benefit from promising space-time techniques. By making use of relay stations (which can accommodate multiple antennas) to relay the message, the channel is effectively converted into a relay channel. Another example is to utilize relay nodes for cooperative communications in ad hoc networks, where the nodes close to the active transmitter and the receiver can relay data packets from the transmitter to the receiver.
FIG. 1 shows a schematic block diagram of a two-hop relaying case as described for example in R. Pabst, B. H Walke, D. C. Schultz, P. Herhold, S. Mukherjee, H. Viswanathan, M. Lott, W. Zirwas, M. Dohler, H. Aghvami, D. D. Falconer, and G. P. Fettweis, “Relay-Based Deployment Concepts for Wireless and Mobile Broadband Radio,” IEEE Communications Magazine, pp. 80-89, September 2004. According to FIG. 1, two nodes S1 10 and S2 20 can communicate with each other via an intermediate relay station (RS) 12 assuming that a direct communication between the two nodes 10, 14 on a single hop H1 is not possible, e.g., due to shadowing or limited transmit power. In general, the RS 12 receives a signal from the first node S1 10 on a first hop H2 and retransmits a signal addressed to the second node S2 14 on a second hop H3.
There are two prominent concepts for this two-hop relaying case: amplify-and-forward (AF) and decode-and-forward (DF), as described for example in J. N. Laneman, D. N. C. Tse, and G. W. Wornell, “Cooperative Diversity in Wireless Networks: Efficient Protocols and Outage Behavior,” IEEE Transactions on Information Theory, vol. 50, no. 12, pp. 3062-3080, December 2004. In AF, the received signal at the RS 12 is only amplified and retransmitted by the RS 12, whereas in DF, the received signal is decoded, re-encoded, and retransmitted by the RS 12.
Assuming that the RS 12 can only receive and transmit on orthogonal channel resources, the required resources in AF relaying for instance are doubled compared to a direct communication between two nodes, i.e., although two-hop relaying aims at an increase in spectral efficiency there exists a trade-off in that a conceptual degradation of the spectral efficiency by the factor ½ is inherent.
Several proposals have been made to compensate this conceptual drawback of two-hop relaying. Common to all these protocols is that it is not possible to improve the spectral efficiency of a single two-hop connection between source and destination, but the overall spectral efficiency of different two-hop connections. In H. Shi, T. Asai, and H. Yoshino, “A Relay Node Division Duplex Relaying Approach for MIMO Relay Networks,” in Proc. of IEEE 17th Symposium on Personal, Indoor and Mobile Radio Communications, Helsinki, Finland, September 2006, several two-hop connections with multiple RSs are considered. These RSs are divided into two groups. While the first group of RSs receives signals, the second group of RSs transmits signals on the same channel resource and vice versa. This protocol significantly increases spectral efficiency of the network.
Two other protocols are introduced in B. Rankov and A. Wittneben, “Spectral Efficient Signaling for Half-duplex Relay Channels,” in Proc. Asilomar Conference on Signals, Systems and Computers, Pacific Grove, November 2005 and B. Rankov and A. Wittneben, “Achievable Rate Regions for the Two-way Relay Channel,” in Proc. IEEE Int. Symposium on Information Theory (ISIT), Seattle, USA, July 2006. The first protocol is called “two-path relaying” and the second protocol is called “two-way relaying”. The principle of the cooperative protocol “two-way relaying” is based on the framework of network coding proposed in R. Ahlswede, N. Cai, S. R. Li, and Yeung R. W., “Network Information Flow,” IEEE Transactions on Information Theory, vol. 46, no. 4, pp. 1204-1216, July 2000, where data packets from different sources in a multi-node computer network are jointly encoded at intermediate network nodes, thus saving network resources. For two-way relaying, two nodes transmit simultaneously on a first channel resource to a RS which receives a superposition of both signals. On the second channel resource, the RS retransmits this superposition. Due to the broadcast nature of the wireless channel, both nodes receive that superposition and may detect the desired signal from the other node by subtracting their own known signal. It can be shown that the spectral efficiency of two-hop relaying is significantly increased by the two-way relaying protocol.
However, in order to design an adequate receive filter both nodes of the two-way relay network require channel state information (CSI) about their own link to the RS as well as CSI about the link from the other node to the RS. Exchanging this CSI requires a high signalling effort.